1. Field of the Invention
This invention is directed generally toward a centrifugal separator and more particularly pertains to an apparatus for providing an evenly-distributed slurry feed to a filtering basket of the centrifugal separator.
2. Description of the Related Art
A centrifuge is a machine which uses centrifugal force for separating substances of different densities such as liquids and solids contained in a slurry mixture. In a filtering-type centrifuge a slurry feed is introduced to a filter basket rotating at a high angular velocity. The centrifugal force generated by the rotating basket removes the liquid components of the slurry from the solid components. The liquid components are forced to flow through perforations in the filtering basket while the solid components are retained on a filtering media placed inside the basket. The remaining solid components on the filtering basket are referred to as a cake.
Typically, most feed slurries have been injected into a filtering-type centrifuge through a stationary pipe. Such an operation relies upon the liquid nature of the slurry and the centrifugal force generated by the rotating basket to level the solid components which are collected in the basket. This leveling action helps prevent ridges from forming in the cake. Ridges are undesirable because they tend to imbalance the basket causing vibration stress to be introduced to the entire centrifuge system. Further, the ridges make subsequent washing of the cake very difficult. In the case of a very fast dewatering slurry, the solids are deposited at the point of discharge from the stationary pipe and therefore are not distributed evenly because the liquid which would normally carry them throughout the basket simply filters away too quickly. There is a long-felt need in the art for a method and apparatus for providing an evenly distributed liquid-solid slurry feed to a filtering basket centrifuge so that an even cake is created on the filtering media without significant ridges.
In the past, several different methods have been used to attempt to provide such an evenly distributed liquid-solid slurry feed to a filtering basket centrifuge. One inexpensive prior art solution is a multi-ported pipe installed in the centrifuge. For example, instead of having only one discharge point, three or four openings are provided in the pipe from which discharge occurs. Thus, the solids are distributed at various locations along a basket surface. A problem occurs with a multi-ported feed pipe-type system when such a pipe is installed in a vertical basket-type centrifuge. Since the slurry is fed from all openings simultaneously, a greater flow mass occurs at the bottom openings than at the top due to gravity. In order to compensate tier this, the size of the lower openings have to be reduced to allow more flow mass to be apportioned through the upper openings. Due to the decrease in size of the lower openings, the velocity of the slurry leaving the bottom openings is greater than the velocity of the slurry as it leaves the top openings. An increased velocity at the bottom tends to increase the chance that the feed will splash backwards. Such backward splash tends to cause the liquid/solid slurry to exit the basket through the spoked bottom. This contaminates clean, washed cake that may have been discharged during prior batches. In order to compensate for the increased possibility of splash, flow velocity must be kept at a very low rate. This introduces problems because control methods for regulating velocities must be added to the system. In some situations, For example, because of very high flow rates or very dense slurry mixtures, it may not even be economically feasible to attempt to regulate velocities.
Another attempt at providing an evenly-distributed slurry feed to a filtering basket centrifuge was made by utilizing a multi-pipe type of arrangement that is grouped together in a manifold. The manifold typically has a common oversized inlet and all of the pipes are of the same diameter so that the output velocity is held common. The multi-pipes help to evenly deposit solids in the centrifuge basket by cutting down the distance that the solids have to travel. The primary disadvantage of such a multi-pipe feed device is the expense of providing such a manifold and the difficulty of fitting such a manifold device into a centrifuge.
Either the multi-ported feed pipe or the multi-pipe manifold discussed above can also be adapted with an oscillating mechanism to force up and down, or back and forth movement, to further help distribute the solids. The primary disadvantage of such an oscillating device is the expense of providing such a mechanism and the tendency of such a device to be mechanically unreliable due to wear.
Another type of device which has been implemented to attempt to provide an even distribution is an angled-rotary feed cone. The device consists of a variable speed drive and a two-part angled feed cone which is rotated by the drive about a stationary feed pipe. The feed pipe and the cone are placed inside the basket of a centrifuge. The feed pipe and drive shaft are in an extended position. The bottom half of the cone is attached to the drive shaft and has no opening. The feed from the pipe is deposited on the bottom half of the cone. The two halves of the cone are attached to each other by means of small cylindrical spacers at the outer perimeter of the cone. The spacers have gaps in between them. The slurry exits the assembly through these gaps. As the cone is rotated, the slurry that strikes the bottom cone is flung out in a full circle between the two cones. This creates a relative top-to-bottom painting motion between the feed slurry and the basket due to the angle of the cone and also to the difference in rotational speeds between the basket and the feed cone. The feed cone is effective for enabling an even distribution of the slurry on a basket; however, it is quite expensive to manufacture, operate and maintain, and in fact, it is the most expensive overall of the devices discussed thus far.
U.S. Pat. No. 648,088 discloses a centrifugal concentrator including a feed governor that is used to regulate the feed rate of a material entering a separating casing. The feed governor has an inner pipe with a round hole that is located inside of a floating outer pipe which has an inverted triangular opening so that the wider part of the triangle is above the apex of the governor. When the outer pipe is in the fully down position the wide portion of the triangle aligns with the hole in the inner pipe forming a dual port. This allows for the highest flow slurry through this dual port. An increase in pressure caused by the accumulating volume of the slurry pushes the outer pipe upward. This upward movement of the outer pipe decreases the size of the feed opening because the triangle is moved in alignment with the port. The blockage of the ports slows the rate of feed into the separation casing. The net result is that the inflow into the casing is roughly equal to the outflow of material out of the casing. The disclosed feed governor is designed to vary feed flow rate at a single discharge point on a feed pipe. Because there is a single feed point there remains the possibility with this type of device that solids may accumulate at one point.
A three-pipe system is disclosed in U.S. Pat. No. 2,648,568 that uses a centrifugal pipe pump for slinging out material, such as paint, in a tangential direction to the pipe transporting the material. The device includes a stationary-slotted innermost pipe, a small diagonally-slotted rotating secondary pipe, and a stationary-slotted outer pipe. The two slotted pipes are positioned so that the slots are not aligned with each other. A liquid material, such as paint, herbicide, or pesticide is fed from a self-contained reservoir into the innermost stationary pipe. The rotating secondary pipe picks up a small amount of the feed material when it passes the inner pipe slot. When the secondary pipe rotates past the slot of the outer pipe, the centrifugal three induced by the rotational motion slings the liquid medium outward in a tangential direction to the rotating pipe. This device takes advantage of the centrifugal motion and the alignment of slots acting together to pump the liquid medium to an intended surface. Unfortunately, this device applied in a filtering centrifuge would often times plug due to the small width of the rotating slot and the large particles that are typically fed to the centrifuge. If the slot was made larger the efficiency of the device as a pump would decrease. Furthermore, the three pipe system of this device would have a tendency to shear the particles as they were forced between the inner and outer pipe. This particle degradation would make filtering more difficult as well as devalue the final product in many cases.
An electrostatic atomizer of liquids which creates a mist-like flow of a finely dispersed liquid exiting an outer slot is disclosed in U.S. Pat. No. 2,695,002. This patent describes many variations of an inner helical groove or series of inner helically-positioned holes which feed an electrostatically-charged liquid through a straight slot or straight series of perforations. The relative motion of the inner tube to the stationary outer tube creates the mist-like flow of the electrically-charged liquid. Because of the electrical arrangement, the electrostatic atomizer has a feed inlet on one end and a drive on the other end. However, it is impractical to implement a mist-like flow given the large particle size, high velocity, and solids concentration of the slurry in a centrifuge.
Another spraying device which attempts to provide uniform distribution is disclosed in U.S. Pat. No. 2,994,482. This device is designed for the distribution of gas and liquid to contact apparatus such as elements of a cooling tower. The disclosed spraying device has a rotating outer tube with a helical spiral of perforations and a stationary inner pipe with a straight slot. The device sprays in one direction only and the spray travels longitudinally from one end to the other in one revolution. Then the spray is forced back to the opposite end and begins its travel again. Such an arrangement is ineffective for achieving a uniform distribution because the medium is unevenly applied due to an uneven jumping motion created by the abrupt change from one end to the opposite one without spraying points located in-between.
The inability to apply a medium in an uninterrupted fashion is an inherent disadvantage of both the disclosed container rinsing apparatus of U.S. Pat. No. 3,136,324 and the spray spout for use in a dishwashing machine disclosed in U.S. Pat. No. 3,146,953. Both of these devices employ grooves and rotational elements to attempt to distribute liquids in a uniform fashion but also share the disadvantage of spraying liquid in one direction only, and then jumping back to the opposite end without hitting points in-between following the direction change.
U.S. Pat. No. 3,348,767 discloses an entire centrifugal separator having a combination open screen area for the passage of a wash liquid. The feed portion of the process occurs in the sedimentation area of the basket. The disclosed centrifuge includes a reciprocating wash pipe that pours wash liquid into various dam segments of a large hollow cylinder by using a lengthwise reciprocating sliding movement. The liquid is discharged in a full 360.degree. circle. Such a device would be ineffective in a filtering basket-type centrifuge because the basket is already rotating. Further, the reciprocating motion of the device tends to cause undesirable vibration and increases mechanical wear.